Polypropylene is known to have excellent heat resistance, chemical resistance, electric properties, rigidity, tensile strength, optical characteristics and is also known to process easily. Crystalline propylene-ethylene resins are known to mold easily and have been broadly used for injection molding, film- or sheet-extrusion molding, blow molding, and like applications.
Rubber modified polypropylene resins produce articles which generally exhibit a desirable balance of such properties as melt strength, flexural modulus, yield stress and elongation, tensile stress, N-izod, Gardner Impact, heat distortion temperature values, and the like. Unfortunately, polypropylene articles are also known to exhibit surface imperfections, such as flow marks and silver streaks. Silver streaks are generally associated with an overheating phenomena while flow mark imperfections appear to be associated with the viscosity, or flowability, of the resin.
Flow marks manifest on the surface of injected molded articles as a series of alternating high and low gloss chevrons. The general trend of each band is approximately perpendicular to the direction of melt flow during injection. Flow marks are not unique to rubber modified polypropylene since homopolymers also display the phenomena under various molding conditions. While these marks do not perceivably affect mechanical properties of the molded article, nor are they discernible by touch, nonetheless their presence gives the appearance of inhomogeneity within molded parts reducing their aesthetic appeal. The effect is pronounced in large samples with a high aspect ratio, such as automobile parts, for example, dashboards and bumper covers.
The art has attempted to reduce flow marks in molded articles by decreasing the viscosity of the polypropylene resin. This technique decreased the appearance of the flow marks, however the decrease in viscosity also modified other physical properties. Flow marks may also be decreased by annealing the article after the molding process. This annealing step however is not commercially feasible or desirable in view of the increased energy required to anneal the article, extended time to anneal, and equipment modification necessary to allow injection molding apparati to also serve as annealing mediums. Adding fibrous or granular fillers, such as calcium carbonate or glass fibers to polypropylene compositions tends to improve rigidity of the molded article, however the surface appearance tends to be poor and these fillers give rise to warpage and deformation in the resultant article. The art has also described adding rubber components to polypropylene to improve the appearance of the resultant injection molded or welded articles. However, the art has concentrated on use of thermoplastics or high molecular weight solid components to improve the general appearance, coatability, and weldability of the resultant molded articles.
Numerous methods have been attempted to decrease flow marks in polypropylene resin compositions, however, these methods have failed to improve the appearance of the resultant article without sacrificing desirable physical properties. Thus, there exists a need to reduce the flow marks in articles produced from polypropylene resins, without affecting other physical properties.